Rashi seems to have combined two understandings of māsōret (Ezek. 20:37): “restraining fetter (mōsēr)” and “divine promises God had handed down (māsar).” “Fetter” looks backwards in the verse to God's yoke of obligations and forwards in the verse to the rewards of “the inherited covenantal promise”. Rashi's co-ordination of dual roots in his brief paraphrase seems to anticipate what modern scholars call “Janus parallelism”.
Ezekiel 20:1–37a speaks of how God chose Israel for himself, brought them out from Egypt where they polluted themselves with idols and God was angry with them and would have destroyed them. Yet, he gave them laws and festivals which they rebelled against and so he, in time, scattered them amongst the nations. Now he will bring them out, ruling over them with fury (20:33), from the lands of their captivity as he had before into a desert-like state. And he will (20:37) bring them once more under his rod and to his covenant, purifying them (20:38ff) of the rebels amongst them that they may thrive and return to their land.
Ezek 20: 37 is the transition point that moves the narrative from Israel’s past failure and accompanying punishments into a vision of a restored covenant. God will thus repair the breaches that had occurred from the Exodus until the Exile.
I wonder if Rashi’s comments on Ezek 20:37 disclose an intuitive grasp of “Janus parallelism.” This term describes a feature in Scriptures that allows reading verses both in parallel and as progressions. It depends upon the author placing an ambiguous word-form with dual etymologies into the biblical passage. C. H. Gordon coined the term “Janus parallelism” to describe a literary phenomenon “that hinges on the use of a single word with two entirely different meanings: one meaning paralleling what precedes, and the other meaning what follows.”1 The argument that Rashi noted a Janus parallelism runs as follows.
Ezek 20:37: וְהַעֲבַרְתִּ֥י אֶתְכֶ֖ם תַּ֣חַת הַשָּׁ֑בֶט וְהֵבֵאתִ֥י אֶתְכֶ֖ם בְּמָסֹ֥רֶת הַבְּרִֽית
Rashi: שתהיו כפופים לי ולמוסרי - תחת השבט
בברית שמסרתי לכם - במסורת הברית
Rashi’s understanding: I [have you pass] beneath the scepter [and I will bring you]: that you become subject to (a) me and to (b) my band(s) [מוֹסְרַי];”2 [I will bring you] by the masoret of the Covenant [commandments]: By the covenant [commandments] that I handed to you [but you ignored].
The scepter is the symbol of the king’s authority and his laws—so Rashi reads (comment on 37a) as if 37a and 37b are parallel—by making you kneel beneath the scepter you are now my subject and bound by my laws. So Rashi’s first comment reads מָסֹרֶת as obligation (the extended meaning of fetter).3 Greenberg got this translation exactly right although he need not have justified it by seeing מָסֹרֶת as מַאֲסֹרֶת when מוֹסֵר (fetter, band) works more elegantly, with no need for hypothesizing a dropped א.4 For all intents and purposes, they mean the same thing.5 For Rashi the entire verse describes a kind of knighting ceremony where the subject kneels beneath the ruler’s scepter—a ceremony of loyalty to God and his obligations. Rashi’s wording suggests he sees the preposition בְּ prefixed to מָסֹרֶת as carrying a meaning often associated with the preposition לְ (i.e., לְמוֹסְרֵי), namely identifying obligations as the object of one’s loyalty. In his comments to 37a Rashi paraphrases the entire verse.
But then Rashi reads the verse anew: 37b does not mirror back 37a but moves the action ahead: I will bring you [to full redemption] by means of the covenant that I have already delivered to you. Here בְּ signifies agency, “by the covenant.” The inheritance of the laws or of the covenant of land (I’m not sure which he has in mind) are not the just for the purpose of my choosing you to be my people but act also as the instrument by which I lead you forward. And you already have it—I gave it to you long ago. What makes this reading fortuitous is that it could serve as a subtle polemic to undermine any notion that Ezekiel refers to a radically new covenant, as Christians may see it. Rather, he speaks of a renewed covenant, one which was given once and for all time at the occasion of the Exodus. For Rashi, מָסֹרֶת looks backward in the verse to the “scepter of authority” (שֶׁבֶט) and forward to the eternal “covenant” (בְּרִית the next word after מָסֹרֶת) already in hand.
 See C. H. Gordon, “New Directions,” Bulletin of the American Association of Papyrologists 15 (1978), pp. 59–66 (59–60), available online at http://quod.lib.umich.edu/b/basp/browse.html. For a discussion on this device on Job and numerous other texts see Scott B. Noegel, Janus Parallelism in the Book of Job (JSOTS, 223; Sheffield: Sheffield Academic Press, 1996).
 It is possible that Rashi’s מוסרי stands for מוּסָרִי “my instruction,” but such a reading of Rashi would change very little in terms of my argument about the presence of a Janus parallelism.
 Ancient sources note two senses here:
1. “Shut up” (δεσμοῖς)—Aquila (like “in an iron collar,” διὰ κλοιοῦ in Symmachus) While Yehuda ibn Hayyuj proposes this meaning on the basis of an hypothetical reconstruction of a missing א in מַאֲסֹרֶת, most likely these versions took it from מוֹסֵר (band, fetter). On ibn Hayyuj’s proposal see David Kimhi’s ספר השרשים “Book of Roots,” under the heading of אסר and M. Greenberg, “MSRT HBRYT, ‘The obligation of the covenant,’ in Ezekiel 20:37,” in Carol L. Meyers and M. O’Connor, eds., The Word of the Lord Shall Go Forth: Essays in Honor of David Noel Freedman in Celebration of His Sixtieth Birthday (Winona Lake, 1983), pp. 37–46 (39) and bibliography cited there.
2. Another meaning found in an ancient OG version is “handed-down tradition” (παραδόσει)—Theodotion. Rashi incorporates these two meanings in his comments and find both senses embedded in the verse.
 See BDB 64a–b; under main heading אסר, subheading מָסֹרֶת which notes this usage of “moser” מוֹסֵר in Ezek 20:37.